1. Field of the Invention
The present invention relates to an impedance-matching switching circuit that performs impedance matching at a point at which high frequency circuits or elements having different impedances are connected to each other, and an antenna device, a high-frequency power amplifying device, and a communication terminal apparatus that are equipped with the impedance-matching switching circuit.
2. Description of the Related Art
In recent years, communication terminal apparatuses, such as mobile phones, may require compatibility with not only communication systems, such as a GSM (registered trademark) (Global System for Mobile communication), DCS (Digital Communication System), PCS (Personal Communication Services), and UMTS (Universal Mobile Telecommunications System), but also communication systems, such as a GPS (Global Positioning System), a wireless LAN, Bluetooth (registered trademark), and the like. Thus, antenna devices used for such communication terminal apparatuses are required to cover a wide frequency band of 700 MHz to 2.7 GHz.
Generally, antenna devices have a configuration in which the characteristic of an impedance matching circuit is changed by using an active element in order to cover a wide frequency band. As disclosed in Japanese Unexamined Patent Application Publication No. 2008-035065, for example, antenna devices that connect a frequency switching circuit (a frequency switching type matching circuit) including a variable capacitance element at a feeding end of an antenna element are known.
However, small antenna elements used especially for communication terminal apparatuses have extremely large frequency characteristic of impedance, which causes a problem.
FIG. 1A illustrates an example of an antenna device equipped with an impedance matching circuit using an active element. In addition, FIG. 2A illustrates an example of impedance of a small monopole antenna that resonates around 1300 MHz.
The real part R of the impedance of this antenna is:
around 900 MHz: R=6 Ω
around 1300 MHz: R=18 Ω
around 1900 MHz: R=25 Ω.
Monopole antennas ideally have impedance of 23Ω. However, as compared with a wavelength of a high frequency radio signal, as an electrical length of a radiating element is shorter, that is, as antennas are made smaller, impedance is reduced correspondingly. Therefore, in accordance with the wavelength of a high frequency radio signal transmitted and received by this radiating element, the real part of the impedance of antennas is very largely changed.
FIG. 2B illustrates a characteristic of return loss (S11) of this small antenna. In FIG. 2B, AO represents a characteristic in a state in which the series active circuit of FIG. 1A is not inserted, AL represents a characteristic in a state in which the series active circuit has chosen an inductance element of 13 nH, and AC represents a characteristic in a state in which the series active circuit has chosen a capacitance element of 0.65 pF.
In this way, when a series inductor is inserted in a portion of the series active circuit, the resonant frequency is shifted to a lower frequency, and the resonant frequency is shifted to a higher frequency when a series capacitor is inserted.
However, as illustrated in FIG. 2B, even if a reactance element is simply loaded in series so that an imaginary part jx of the inductance becomes zero, a degree of impedance matching (depth of return loss) changes depending on a frequency because the value of a real part R of the impedance of an antenna largely changes depending on the frequency.
Accordingly, since proper impedance matching cannot be performed only in a series active circuit, as disclosed in Japanese Unexamined Patent Application Publication No. H06-053770, for example, it is necessary to perform matching by connecting an inductor and a capacitor in parallel. However, since the value of the reactance that is loaded in parallel has a different optimum value with respect to each frequency, a circuit to be connected in parallel needs to be an active circuit as illustrated in FIG. 1B.
Therefore, at least the following problems occur:
(1) increase in cost due to complications of a matching circuit including an active element;
(2) increase in overall size due to a too large number of elements defining a matching circuit including an active element; and
(3) distortion easily generated due to high load power applied to an active element by loading the active element in parallel.
The above-stated problems generally occur in not only a circuit that performs matching between a feeding circuit and an antenna, but also a circuit that performs impedance matching between two high frequency circuits mutually having different impedances.